How Tube Amps Work

Put your hand in front of an empty electric
socket, and you won’t get a shock—
because electrons just don’t fly through
space, right? Well … they will under the
right conditions—like inside a vacuum tube.

Here we’re going to take a look at the inner
workings of standard amplifier circuits—the
tubes, transformers, resistors, and capacitors
that work together to create the amazing tones
that have powered countless songs for the past
60+ years. While this stuff may be daunting to
some of you, take heart—this is century-old
technology. The basic concepts really are not
too difficult to grasp.

We’ll discuss amplifier circuits by looking
at my absolute favorite small amp, a 1960s
Vox AC4. While it’s small and simple, an AC4
actually is not the simplest guitar amp out
there. Unlike Fender’s earliest tweed Champs,
the AC4 has a tone control and tremolo,
which gives us a bit more to talk about.

But before we get started, let’s make it clear
that this article is not in any way encouraging
or equipping you to open up the back of your
amp and start poking around. Make no mistake:
Amplifier circuits, even when unplugged,
contain voltages that can kill you. And if you’re
an amp tech, please excuse any oversimplification
in the discussion—this is a primer for general
consumption, not a compendium of possible
exceptions and anomalous phenomena.

The Vacuum Tube

Cathode vs. Fixed Bias
A Vox AC4, like many amps, is designed to make the power tube’s cathode slightly
positive—a state that is commonly referred to in the guitar universe as cathode
biased. Other amps, instead, put a negative charge on the power tube’s grid. That’s
called fixed bias, and it has a similar effect. Either method causes electrons to stay
put on the cathode until needed.

First, let’s talk about some basic principles
of electricity. An electron—the heartbeat
of electric energy—is a negatively charged
subatomic particle. In a vacuum (i.e., in
the absence of air and matter), an electron
will, in fact, fly through space if attracted
by a sufficient positive charge—because
opposites attract. Experiments conducted
well over a century ago demonstrated that
electrons will not only fly through space,
but they can also be controlled. Scientists
showed that, in a vacuum, electrons flowing
from a heated metal element—the
cathode—and being pulled toward a positively
charged element—the anode—can be
deflected by a magnetic field.

Learn how to control that magnetic field
accurately and, as RCA did, you can display
an image of Felix the Cat on a phosphorescent
surface at the far end of the tube. The
tube used in that case was the cathode ray
tube (aka CRT)—better known today as an
old, pre-LCD/LED/plasma television.

In guitar amps, we’re not that interested
in displaying images with our tubes, but
we’re still very interested in controlling those
electrons—and we can use a guitar to do it.
Picture this: In the center of a tube’s glass
envelope is a cathode. It carries just a slight
positive charge, and it’s ready to release a
gazillion electrons. It’s especially ready if it’s
been heated. Surrounding the cathode is the
anode—although in the guitar universe we
typically call it the plate. The plate carries
a high positive charge that’s ready to pull
those negative electrons toward it. To the
highly positive plate, the cathode’s slight
positive charge still makes the cathode seem
negative (we’ll talk more about this slight
positive charge later). If you place these
two elements in a vacuum and power them
up, electrons will fly relentlessly toward the
plate. When you add a third element—the
grid—between the two, you can control the
flow of electrons. And when you position
the grid close to the cathode and connect
the grid to the relatively tiny voltages coming
from your guitar pickups, something
interesting happens: The tiny signal unleashes
a flood of electrons, allowing them to fly
freely to the plate. That rush of electrons
from the cathode to the plate mirrors the
signal from the guitar, amplifying its signal
many times over.

Okay, so let’s get back to that earlier
mention of the slight positive charge. The
reason we want the cathode to carry a slight
positive charge is that it makes the grid,
with no charge yet applied, seem negative.
Voltages are relative. And while opposites
attract, like charges repel. The apparently
negative grid close to the cathode will keep
those negatively charged electrons in place
until the guitar signal is ready to swing the
grid positive to release them.

One other useful electron-related fact to
know is the difference between voltage and
current. Think of current as the amount of
water flowing through a pipe. More current
means more water being delivered. Voltage,
on the other hand, is like water pressure—
it’s the force behind that water. Increase the
voltage (pressure) and you’ll increase the
current (amount of flow). A resistor acts
like a constriction in the pipe, with more
resistance being analogous to a tighter constriction.
So it follows that placing a different
resistor in a circuit will affect both the
voltage and the current.

What actually goes on inside a guitar
amplifier is obviously a bit more complex
than just the flow of electrons in tubes, though. Next we’ll do a quick overview of the additional parts involved, followed by more detailed, part-by-part descriptions.

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